Delta-sigma modulators are particularly useful in digital to analog and analog to digital converters (DACs and ADCs). Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, a low-pass delta-sigma modulator performs noise shaping by acting as a lowpass filter to the input signal and a highpass filter to the noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta-sigma modulator ADC includes a summer that sums the input signal with negative feed-back, a linear filter, a quantizer, and a feed-back loop with a digital to analog converter coupling the quantizer output and an inverting input of the summer. In a first order modulator, the linear filter comprises a single integrator stage while the filter in a higher order modulator comprises a cascade of a corresponding number of integrator stages. The quantizer can be either a one-bit or a multiple-bit quantizer. Higher-order modulators have improved quantization noise transfer characteristics over those of a lower order, but stability becomes a more critical design factor as the order increases.
In particular, higher order delta-sigma modulators implement multiple sets of poles and zeros that advantageously provide for improved noise shaping and consequently an increased signal to noise ratio (SNR) in the signal base band. However, high order delta-sigma modulators with aggressive noise transfer functions (NTF's) (i.e. NTF's that provide high out-of-band gain) are also subject to instability for large input signals. Generally, in order to avoid instability with higher order delta-sigma modulators, it becomes important to maintain the input voltage range within given limits in order to ensure that the loop filter integrator stages do not saturate and/or that the quantizer does not overload. Additionally, stability can be ensured by decreasing the out-of-band gain of the NTF. Disadvantageously, reducing the input voltage range limits the dynamic range of the delta-sigma modulator, while reducing the NTF out-of-band gain reduces the SNR.
In sum, new techniques are desirable for designing and constructing high order delta-sigma modulators having NTF's with high out-of-band gain and a maximum dynamic input voltage range.